I thought it might be good to post a few images that touch on the visual development of the Sith Lords.

The basic story of how Ralph McQuarrie created the iconic look of Darth Vader is pretty well-known. (In brief: Lucas wanted Vader to enter a depressurized ship cabin in his first scene of the film; so, realizing that Vader needed some sort of breathing mask, McQuarrie designed an armored spacesuit, which looked so awesome that Lucas just had Vader wear the suit all the time.)

Most of McQuarrie's concept art for Vader is widely available enough that I don't think I need to post it here. That said, I do want to highlight a few interesting yet little-seen drawings, which shed a bit of extra light on the design process of Vader and the other Sith Lords (who were ultimately dropped from ANH). Several of these drawings are by costume designer John Mollo--McQuarrie's pretty widely (and justly) acclaimed, but I don't think Mollo's work on SW gets enough attention.

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A John Mollo drawing of Darth Vader, based of course on McQuarrie's "armored spacesuit" design. This concept by Mollo depicts Vader's armor like that of a medieval Black Knight; it doesn't seem to have any obvious life-support functions, though.

The Imperial insignias seen here on Vader's chestplate, helmet and cape are very reminiscent of the armor of black-clad and helmeted villain The Lightning in the 1938 Republic serial The Fighting Devil Dogs. The Lightning's all-black, face-concealing costume had his white lightning-bolt insignia marked on his cape and chest and helmet. (His minions sometimes wore all-white radiation suits... in the fashion of Stormtroopers.)

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(The Lightning is the figure on the far right.)

I also might mention a little detail in the above Mollo drawing of Vader-- in the lower right corner he's sketched a three-pronged triskelion (basically a swastika with three arms). It's probably a doodle of a potential Imperial insignia, and it bears rather a good resemblance to the three-armed lightning bolt insignia of The Lightning.

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Another Mollo drawing of Vader. This one dates to January 15, 1976, so it was influenced by Lucas's January 1 fourth-draft script, which made Vader the sole Sith Lord in the film. Here Vader, not Obi-Wan Kenobi, is the major cyborg character.

Mollo's take on the chest plate is interesting and rather different from McQuarrie's chest plate design (which was closer to the final film version). I understand the motif here was reworked and used for the white CZ-3 droid in the Jawa sandcrawler.

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A third Mollo drawing of Vader, from the same point in production as the one just above. It clearly shows another variant of Mollo's chest plate design. (The placement of the cyborg life-support tubing is slightly different than in the other drawing.)

Also, the hood on Vader's cape is interesting--Mollo seems to have imagined that Vader could wear a hood over his helmet.

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A circa-1975 drawing by Ralph McQuarrie of a "dark knight," presumably a Sith Lord. This concept design later wound up influencing the armor of Boba Fett in ESB.

Here's another version of Ralph McQuarrie's armored Sith Lord concept drawing for ANH, in which the armor color was white like Stormtrooper armor:
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Although Lucas's earliest notes for Draft 2 of ANH describe the Sith knights as "look[ing] like Linda Blair in The Exorcist," similar to how the Emperor ultimately looked in ROTJ, the subsequent ANH Draft 3 mentions that the Sith Lords (besides Vader) wear helmets. "The first Sith Lord puts his hand to his ear as a message comes through on his helmet intercom."

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A John Mollo drawing showing at left, a TIE fighter pilot, and on the right, a Sith Lord. It appears the other Sith were envisioned as being very similar to Vader in the design of their armor. (McQuarrie's version is a little less similar and more like a stormtrooper.)

Note the two small horns on the sides of the Sith Lord's helmet; at one point Mollo experimented with drawing similar horns on Vader's own helmet.